Electrolytic descaling



United States Patent ELECTROLYTIC DESCAL ING John J. Dailey, Arlington, Tex., assignor to Temco Aircraft Corporation, Dallas, Tex.

No Drawing. Application August 5, 1955, Serial No. 526,763

7 Claims. (Cl. 204-141) This invention relates to a method for descaling titanium and its alloys which have been exposed to heat, and more particularly to a method of removing oxide coatings from hot formed or stress relieved sheet metal parts fabricated from titanium and its alloys.

In the manfacture and fabrication of titanium articles which require heat during the forming operations or a stress relieving treatment, unless costly inert atmospheres or chemical retardants are provided, oxides of titanium are formed on the exposed surfaces of the metal. Such oxidized titan um parts then require further processing to remove the oxide scale. This descaling process has been very costly when the metal surfaces of satisfactorily fabricated parts became severely etched, pitted, discolored, or they lost gage thickness which resulted in the ultimate rejection of the fabricated part.

At least three methods have been used in the past for the removal of oxide scale from titanium, including in1 mersion of the scaled titanium part in a nitri-hydrofluoric acid bath, the use of a molten salt bath, and various forms of abrasion.

Inasmuch as the action of the nitric-hydrofluoric acid bath is greatly retarded by oils, greases, stamping inks, and many other materials used during the fabrication of titanium parts, appreciable manual effort has been required before immersion. Moreover, due to variations in the oxide thickness caused by uneven heating, severe etching of the titanium surfaces and a reduction in gage thickness has occured unless provisions were made for short time immersions of the article in the nitric-hydrofluoric acid bath followed by hand scrubbing of the part with steel wool. Consequently, parts formed with closed angles or having portions inaccesible to hand scrubbing will never be completely descaled.

The molten salt bath method involves the use of a composition composed essentially of the salts of the alkali metals, heated and maintained at a temperature above 700 F. and usually at a temperature ranging from 800 F. to 900 F. Close temperature control is essential, since a bath too cool reduces the descaling reaction rate, and a bath too hot may ignite the metal. Titanium parts descaled in the molten salt baths are customarily discolored upon removal and require additional chemical treatment for cleaning and brightening of the metal. This chemical treatment usually consists of an immersion in a sulfuric acid bath followed by an immersion in a nitric-hydrofluoric acid bath. Thus, the disadvantages of the molten salt bath include its high original cost, high operational cost, close temperature requirements, and the necessity for further chemical treatment to obtain a satisfactory surface.

The abrasion methods for the removal of scale frequently employed for forgings or heavily scaled parts employ grit, vapor blasting, or grinding, imparting to the article a matte or scratched surface and often a discolored appearance.

It is among the objects of this invention to provide an improved method for descaling titanium and its alloys,

2,780,594 Patented Feb. 5, 1957 2 avoiding the disadvantages and shortcomings of the prior art, resulting in the complete removal of the oxide scale from heat-affected titanium products without materially reducing the metal thickness or severely etching the exposed metal surface, and thereby producing an oxide free titanium surface of uniform appearance, requiring a minimum of manual labor.

It is also among the primary objects of the invention to provide a method of removing oxide coatings from an article of titanium and its alloys comprising immersing the article as a cathode in an electrolytic bath and passing a direct current between the article and an anode, the bath consisting essentially of about 1% by volume of a 48 to 70% solution of hydrofluoric acid, about 4% by. volume of 38-46 B. nitric acid, about by volume of 60-66" B. sulfuric acid and 3 to 5 ounces of position whereby titanium articles of substantially any size or configuration can be freed of oxide scale which forms in the temperature range of 400 F. to 1300 F. The extent of .the oxide removal is such that the article is rendered clean and uniformly bright in appearance Without severe etching, undue gage loss, or costly hand work.

The process is continued until a visual inspection shows the part to be free of oxide scale or the remaining scale free enough to be removed by a subsequent running water rinse with a minimum of manual labor.

The time required for descaling a given titanium article, with this invention, is variable and will depend on the current density, oxide scale thickness, and bath concentration.

When the part appears to be free of scale or the scale loosened sufficiently to be removed by a water rinse, the part is disconnected from the current source, removed from the electrolytic bath, and rinsed in a running water rinse bath.

Titanium parts not completely freed of scale after the running water rinse may be returned to the electrolytic bath, connected to the current sources as the cathode, and the process continued or repeated until a clean, bright oxide-free surface is obtained in the water rinse both.

In practice, the titanium article to be descaled is connected to the cathode lead from a direct current source and immersed in an electrolytic solution composed as follows:

Material: Concentration Hydrofluoric acid (48-70%) 1% by volume. Nitric acid (3846 B.) 4% by volume.

Sulfuric acid (60-66 B.) 20% by volume.

Water by volume. Ferrous sulfate or aluminum 3-5 02/ gal. of other sulfate. ingredients.

The sulfuric acid and sulfate serve to inhibit the reaction of the solution on the base metal and prevent the formation of black smut after the oxide scale has been removed.

A second electrode of titanium or ferrous material is suspended in the electrolyte as an anode of the circuit during the initial flow of current at a direct current voltage of from 6 to 36 volts for a period of from one to three minutes. The current direction may then be reversed for a period of from one to three minutes to facilitate the removal of the scale. The polarity of the current is then reversed to provide current flow in the original direction and the process is continued with the titanium part as the cathode until the oxide scale is loosened or actually removed. The cleaned titanium part is then disconnected from the current source, removed from the electrolytic bath, and immediately rinsed in a running water bath at room temperature. The parts thus thoroughly rinsed, are air dried and inspected for freedom of scale. Any part not completely free of scale may be returned to the electrolytic bath and the process continued or repeated until the rinsed part is bright and free from oxide scale.

Titanium articles have been allowed to remain in the working solution of the present invention for periods as long as one hour without any excessive etching or detrimental gage loss.

Although only one embodiment of the invention has been described, such variations as would suggest themselves to those skilled in the art are contemplated Within the scope of the appended claims.

I claim:

1. A method of removing oxide coatings from an article of titanium and titanium base alloys comprising immersing said article as a cathode in an electrolytic bath and passing an electric current between said article and an anode, said bath consisting essentially of an aqueous solution of hydrofluoric acid, nitric acid, sulfuric acid and a sulfate selected from the group consisting of the ferrous sulfate and aluminum sulfate.

2. A method as defined by claim 1 in which the hydrofluoric acid is a 48 to 70% solution, the nitric acid is a 3846 B. solution and the sulfuric acid is a 6066 B. solution.

3. A method as defined by claim 1 in which the proportions are by volume about 1% hydrofluoric acid, about a of nitric acid, about 20% of a 60-66 B.

4% nitric acid, about 20% sulfuric acid; and from 3 to 5 ounces of the sulfate is present per gallon of the other ingredients.

4. A method as defined by claim 1 in which the anode is a metal selected from the group consitsing of the ferrous and titanium metals.

5. A method as defined in claim 1 in which the current is supplied in one direction for at least one minute, then reversed for from two to three minutes and then again supplied in the original direction until the oxide coating becomes loose.

6. A method of removing oxide coatings from an article of titanium and titanium base alloys comprising immersing the article in an electrolytic bath and subjecting the same to an electric current at a voltage between 6 and 36 volts, said bath consisting essentially of an aqueous solution of hydrofluoric acid, nitric acid, sulfuric acid, and a sulfate selected from the group consisting of the ferrous sulfate and aluminum sulfate.

7. A method as defined in claim 6 in which the proportions are by volume about 1% of a 48 to hydrofiuoric acid solution, about 4% of a 38-46 B. solution Solution of sulfuric acid; and from 3 to 5 ounces of said sulfate per gallon of the other ingredients.

References Cited in the file of this patent UNITED STATES PATENTS 532,394 Rawson Jan. 8, 1895 2,115,005 Blaut Apr. 26, 1938 2,337,062 Page Dec. 21, 1943 OTHER REFERENCES Metallurgia, vol. 49, No. 295, May 1954, page 256, by A. E. Durkin. 

1. A METHOD OF REMOVING OXIDE COATINGS FROM AN ARTICLE OF TITANIUM AND TITANIUM BASE ALLOYS COMPRISING IMMERSING SAID ARTICLE AS A CATHODE IN AN ELECTROLYTIC BATH AND PASSING AN ELECTRIC CURRENT BETWEEN SAID ARTICLE AND AN ANODE, SAID BATH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF HYDROFLUORIC ACID, NITRIC ACID, SULFURIC, ACID AND A SULFATE SELECTED FROM THE GROUP CONSISTING OF THE FERROUS SULFATE AND ALUMINUM SULFATE. 